Composition for acidizing wells



"accordance with A. P. I. Code 29.

2,751,348 Patented June 19, 195

ice

Harold W. Brainerd, Jr., Tulsa, lind Oil and Gas Company, of Delaware NoDrawing. Application April 9, 1953,

Serial No. 347,828

10 Claims. (Cl. 252-8.55)

Okla, assignor to Stano- Tulsa, Okla., a corporation This applicationrelates to the especially oil-producing formations, penetrated by awell. More particularly, this invention pertains to an improvedcomposition for fracturing and acidizing calcareous oilproducingformations.

In the treatment of an oil-producing formation to increase productivity,acid has often been injected into the formation to enlarge the pores.That process has recently been improved by incorporating in the acidvarious gelling or emulsifying agents which increase the viscosity ordecrease the filtrate rate of the acid so that it does not readilypenerate the pores of the formation when the gelled acid is injectedinto a well. This gelled acid can be injected into a formation only bythe application of a very high pressure which fractures the formationand displaces the gelled acid back through the resulting fracture agreat distance into the formation, thus displacing the acid asubstantial distance away from the well, where it reacts upon thecalcareous material in the formation and enlarges the flow channels. Ina copending application, S. N. 221, 136, filed April 14, 1951, in thename of H. W. Brainerd, Jr., C. R. Fast, and G. C. Howard, now PatentNo. 2,689,009, dated September 14, 1954, an improved method andcomposition for gelling acid solutions and injecting these solutionsinto a well have been disclosed. In that case, the acid is renderedviscous or gelled by emulsification with an oily vehicle containing Batugum. It was shown, for example, that in a composition containing 100parts hydrochloric acid, 12 parts kerosene, and 2 parts Batu gum byWeight, an emulsion having between 32 and 37 poises, as measured on aStormer viscosimeter at 600 R. P. M., could be produced depending uponthe fineness of grind of the gum-the viscosity varying inversely as theparticle size of the gum. That method and composition have been usedsatisfactorily, but I have now found that the composition thereindisclosed can be substantially improved, i. e., the viscosity can beincreased, the filtrate rate reduced, and the amount of finely-dividedsolids which are incorporated in the emulsion and injected into aproducing formation can be decreased. Viscosities reported herein are,as measured with a Stormer viscosimeter at 600 R. P. M., in It is,therefore, an object of this invention to provide an improvedcomposition for fracturing and acidizing formations penetrated by awell. It is a more specific object of this invention to provide anadditive for mineral acid-hydrocarbon emulsions produced with Batu gumas heretofore used whereby the emulsion can be produced with a lowersolids content. These and other objects of the invention will becomeapparent from the following description.

In brief, this invention comprises a fracturing and acidizingcomposition wherein an aqueous acid solution is made viscous or made tohave a low filtrate rate by emulsification with a hydrocarbon liquidusing Batu gum and a solvent for the gum. By combining a solvent for theBatu gum in the emulsion, it has been found that the amount of gumrequired to produce a stable emulsion of acidizing of formations,

high viscosity and low filtrate rate can be reduced to a small per centof the amount of gum required to produce a similar emulsion without sucha solubilizing agent.

As disclosed in the above-mentioned copending case,- any strong mineralacid solution such as hydrochloric, nitric, or hydrofluoric acid, i. e.,any acid which reacts with the material in a formation and whichproduces a water-soluble salt can provide the base fluid for the presentcomposition. Inasmuch as hydrochloric acid is customarily used foracidizing calcareous formations and is generally available inoil-producing areas, it is generally the preferred base acidizing fluid.The concentration of the hydrochloric acid may vary from about 5 toabout 20 B. A concentration of about 10 B.-15 per cent hydrochloric acidis preferred.

The immiscible liquid with which the strong mineral acid is emulsifiedmay consist of any nonaqueous liquid such as vegetable or mineral oils.Liquid petroleum hydrocarbons, e. g., crude oil, kerosene, diesel fuel,or other light liquid aliphatic hydrocarbons of this class are typical.In view of its wide availabilty and substantially uniform composition,kerosene is the preferred hydrocarbon liquid.

The emulsifying agent, Batu gum, is a natural resin, related to theDamar natural resins, which is a secretion or exudation of the Shoreatree of the East Indies. This gum is insoluble in water, soluble in arylor coal tar hydrocarbons and is hydrogenated aliphatic petroleumsolvents, but is generally only very sligthly soluble in crude petroleumor refined parafiinic petroleum hydrocarbons. It is, however, compatiblewith and can be dispersed in paraflinic hydrocarbons, both crude andrefined. It is available commercially in classes designated as boldscraped, unscraped, nubs and chips, and as dust. Any of thesecommercially-available forms produce satisfactory emulsification of theacid and hydrocarbon phases, providing the particles are suitablyground. In order that the gum may dissolve or disperse readily in thehydrocarbon phase, it is generally ground to pass through a -mesh U. S.standard sieve or finer. Particles of even smaller size are preferredinasmuch as the viscosity of the emulsion has been found to increase asthe particle size of the gum is decreased. Grinding to a particle sizeof less than about 325-mesh sieve is desired.

The Batu gum solvent used to disperse the gum in the hydrocarbon phasemay comprise any material which dissolves the gum or tends to solubilizeit in the hydrocarbon phase. The aryl and coal tar hydrocarbons areexamples. Since heat is sometimes required with these solvents, thesaturated, inert, water-soluble, cyclic monoor diethers having 3 to 4carbon atoms in a 5 to 6 member ring are more desirable. Examples ofsuch ethers are tetramethylene oxide (tetrahydrofuran); trimethyleneglycol methylene ether, 1,3-dioxane (m-dioxane); glycol ethylene ether,1,4-dioxane (p-dioxane); ethylene glycol methylene ether,(1,3-dioxolane); 1,3,5-trioxane (symtrioxane); and diethylenimide oxide(morpholine). All of these solvents are characterized by substantiallyinfinite solubility in water, alcohol, and diethyl ether.

The emulsion is prepared by first mixing with the hydrocarbon phasebetween about 5 and about 15 parts of the solvent per 100 parts byWeight of the hydrocarbon. Typically, about 8 parts ether solvent per100 parts kerosene are used. After the ether solvent has been dissolvedin the kerosene, between about 2 and about 6 parts of the finely-dividedBatu gum per 100 parts by weight of liquid hydrocarbon are mixed intothe hydrocarbon-ether solution. About 4 parts of Batu gum per 100 partsby weight of kerosene are preferred. The gum disperses in thehydrocarbon within a few minutes. This hydrocarbon liquid containing theemulsifying agent and the solvent is then mixed, preferably at a highshear rate, with the acid solution. The ratio of the nonaqueous phase tothe acid solution may be varied over a substantial range. It is highlydesirable, however, that the acid solution predominate in the emulsioninasmuch .as the acid solution is the effective solvent for theformation or deposit which is being fractured. A small proportion of thehydrocarbon phase, typically between about and about parts per 100 partsby weight of the acid solution, is used. In the preferred embodimentabout 3 barrels of hydrocarbon (126 gallons) are emulsified with about1,000 gallons of per cent hydrochloric acid solution.

The two phases may be emulsified by any number of means. A homogenizeris preferred. The viscosity of the emulsion depends somewhat upon themethod used to produce the emulsion. Using even the very smallconcentrations of Batu gum, as above stated, it has been found that aviscosity as high as 50 poises may be obtained even when the two phasesare emulsified with an ordinary reciprocating pump. Higher viscositiescan be obtained using jet-type homogenizers and the like. Gen erally, aviscosity in the range of a few hundred to five thousand centipoises, asmeasured on the Stormer viscosimeter at 600 R. P. M., is consideredsufficient to fracture most formations. A liquid having a viscosity aslow as 30 centipoises or having a filtrate rate of less than about 100cubic centimeters per 30 minutes in the standard A. P. I. filtrate ratetest is also considered desirable.

A fracturing gel prepared from 100 parts of 15 per cent hydrochloricacid solution, 12 parts kerosene, and 2 parts Batu gum, without specialsolvents, had a viscosity, as measured on a Stormer viscosimeter at 600R. P. M., after mixing for 30 minutes with a high speed mixer of between32 and 37 poises, depending upon the fineness of the grind. Viscositiesare as measured on a Stormer viscosimeter at 600 R. P. M. Specifically,with 200 mesh Batu nubs and chips a viscosity of 34 poises was obtained.By comparison and as an example of the effect of a gum solvent, usingthe same amount and concentration of hydrochloric acid solution andkerosene, the same emulsifying technique, and using only 1 part byweight of p-dioxane, a 50 poise emulsion was prepared with only 0.5 partof the same gum.

In another example, 1.075 parts by weight of Batu gum were dispersed in60 parts kerosene using 5.83 parts of morpholine, and 400 parts of 15per cent hydrochloric acid were then emulsified in this hydrocarbonphase using rapid agitation. The emulsion formed within about 2 minutes,and after about 10 minutes of rapid mixing, the viscosity of theemulsion on a Gardner mobilirneter measured 1500 centipoises. The fluidloss of the emulsion using the standard A. P. I. filtrate rate apparatusdescribed in Code 29 measured 85 cubic centimeters in 30 minutes. Forcomparison the viscosity of the emulsion on a Stormer viscosimeter at600 R. P. M. measured 335 centipoises. A similar test was then conductedwherein no solvent was used. In this test 4.3 parts by weight of theBatu gum (four times the amount of Batu gum previously used) weredispersed in 60 parts of kerosene and 400 parts of 15 per centhydrochloric acid were then emulsified in this Batu gum-kerosenemixture. The viscosity of the emulsion thus produced measured only 850centipoises on the Gardner mobilimeter and 230 centipoises on theStormer viscosimeter. The A. P. I. fluid loss at 100 p. s. i. for 30minutes was 38 cubic centimeters. A similar test was then conductedwithout a solvent wherein the amount of Batu gum was reduced to thatused in the morpholine test described above. Again 1,075 parts of Batugum were dispersed in 60 parts of kerosene, and 400 parts of 15 per centhydrochloric acid were then emulsified in this Batu gum-kerosene mixturewithout the use of a solvent. The viscosity on the Stormer viscosimeterat 600 R. P. M. measured 122 centipoises. This emulsion was too thin tomeasure on a Gardner mobilimeter. The A. P. I. fiuid loss using thestandard procedure was 166 cubic centimeters in 30 minutes.

For comparison, a test similar to that described above but using adifferent solvent, namely toluene, was then conducted. In this case,11.66 parts of toluene and 60 parts of kerosene were mixed together, and1.075 parts by weight of Batu gum were dispersed therein by mixing forabout 2 minutes. Then, 400 parts of 15 per cent hydrochloric acid wereemulsified in this nonaqeous phase after the mixture had been heated toF. The stable emulsion formed within less than about 10 minutes. Theviscosity measured on a Stormer viscosimeter at 600 R. P. M. was 172centipoises. The A. P. I. fluid loss using an acid resistant number 52,filter paper was 63 cubic centimeters in 30 minutes.

Thus, even when the amount of gum was reduced to only A that required inthe nonsolvent test, other things being equal, a substantial increase inviscosity was obtained by adding a small but effective amount of a gumsolvent to the nonaqueous phase.

This gel or emulsion may be prepared at a central point and stored forlong periods of time since the acid does not readily attack or destroythe emulsifying agent. In practice, the emulsion is, however, generallyprepared at a central bulk station on the day it is used. Thisfracturing liquid is then transported in tank trucks or the like to thewell site. High pressure, high capacity pumps displace the emulsion downinto the well in accordance with the procedure described in U. S. Patent2,596,843 to Farris. When it hasbeen displaced back into the calcareousformation by the application of a pressure sufiicient to fracture theformation, the emulsion is partially broken. The acid solution thuscontacts and reacts with the calcareous material therein producing awater-soluble salt. This reaction of the acid with the formation appearsto break the acid-in-oil emulsion and release substantially all of theacid for reaction with the formation. While it can readily be shown thatthe gel or emulsion breaks due to contact with a calcareous formation,the real cause of the reversion of the emulsion is not definitely known.The reaction products, the change in pH of the acid, or perhaps acombination of these and other elements may be the cause. The breakdownof the emulsion appears to be very rapid in the presence of a surplus ofcalcareous material. Within about two hours a 50 poise emulsion has beenfound to be reduced to a viscosity of a few centipoises, and withinabout 24 hours the emulsion is completely reduced so that its viscosityis about that of the respective base aqueous and nonaqueous phases.These components then being of about the same viscosity as the wellfluids may be readily removed from the formation by producing liquidsfrom the well.

It can be seen that by the use of a solvent, particularly the cyclicethers, the amount of Batu gum required to produce a very viscousemulsion with an acid solution and a hydrocarbon liquid is reduced to asmall percentage of the amount required where no such solvent isemployed. This reduction in the solids content of the fracturing liquidis, of course, highly desirable inasmuch as the injection of colloidalsolids into a formation has been shown to reduce the permeability of aformation. Accordingly, it can readily be seen that by incorporating asmall but effective amount of liquid solvent in the acid-hydrocarbon gelproduced with Batu gum, the resultant permeability of a fractured andacidized formation is materially increased. It can also readily be seenthat this invention is susceptible of a wide variety of embodiments. Forexample, it will be apparent that various derivatives of the saturated,inert, water-soluble, cyclic ethers above described may be substitutedfor the preferred class and the examples given without departing fromthe spirit of the invention. A propping agent such as sand may also beincorporated in the emulsion. This invention should accordingly beconstrued to be limited only by the scope of the appended claims.

I claim:

1. A composition of matter comprising an acid-in-oil emulsion of astrong mineral acid solution and an aliphatic hydrocarbon liquidcontaining Batu gum and sutficient saturated, water-soluble cyclic etherto disperse said gum in said liquid.

2. A composition according to claim 1 wherein said saturated,water-soluble, cyclic ether is selected from the group consisting ofmonoand diethers having from 3 :to 4 carbon atoms in a 5 to 6 memberring.

3. A well-treating fluid comprising a finely-divided Batu gum dispersedin an aliphatic oily vehicle with a sat urated, water-soluble, cyclicether having 3 to 4 carbon atoms in a 5 to 6 member ring and an acidsolution capable of forming a water-soluble salt from earth formations,said acid being emulsified as the discontinuous phase in said oilyvehicle.

4. An acid-in-oil emulsion comprising 100 parts of said acid and betweenabout 10 and about parts by weight of an aliphatic liquid hydrocarbon,said hydro carbon containing between about 0.5 and about 6 parts offinely-divided Batu gum and suflicient water-soluble cyclic ethers todisperse said Batu gum in said hydrocarbon and produce an emulsionhaving a viscosity greater than about centipoises.

5. An acid-in-oil emulsion comprising by weight parts of a mineral acidsolution and between about 10 and 7 about 15 parts of an aliphaticliquid hydrocarbon by weight, said liquid hydrocarbon containing betweenabout 0.5 and about 6 per cent by weight of finely-divided Batu gum andbetween about 5 and about 15 per cent by weight of a solvent selectedfrom the groups consisting of saturated, water-soluble, cyclic monoanddiethers having 3 to 4 carbon atoms in a 5 to 6 member ring.

6. An emulsion in accordance with claim 5 in which said solvent istrimethylene glycol methylene ether.

7. An emulsion in accordance with claim 5 in which said solvent istetramethylene oxide.

8. An emulsion in accordance with claim 5 in which said solvent isglycol ethylene ether.

9. An emulsion in accordance with claim 5 in which said solvent isdiethylenimide oxide.

10. .An emulsion in accordance with claim 5 in which said solvent isethylene glycol methylene ether.

References Cited in the file of this patent UNITED STATES PATENTS2,038,720 De Groote Apr. 28, 1936 2,045,759 De Groote June 30, 19362,050,932 De Groote Aug. 11, 1936 2,596,137 Fast May 13, 1952

1. A COMPOSITION OF MATTER COMPRISING AN ACID-IN-OIL EMULSION OF ASTRONG MINERAL ACID SOLUTION AND AN ALIPHATIC HYDROCARBON LIQUIDCONTAINING BATU GUM AND SUFFICIENT SATURATED, WATER-SOLUBLE CYCLIC ETHERTO DISPERSE SAID GUM IN SAID LIQUID.